The invention relates to a plastics moulding, in particular a moulded part such as a kitchen sink, a washbasin, or a work-top or similar, comprising a polymer phase formed from a curable reaction mass and a particulate filling material embedded therein, which contains a resin material filled with an inorganic filler. The invention relates in particular to mouldings with a so-called granite design.
Moulded parts and slabs with granite designs or else with plain-coloured surface are manufactured traditionally by conventional casting methods. For this all the components of a casting compound including the fillers are mixed, degassed and cast/pumped into moulds and polymerised. In the case of slabs the continuous belt casting method is also applied on a large scale. Casting compounds with a viscosity in the range from 2000 to 40000 mPas are pourable/pumpable.
In the case of acrylic (or polyester) composites with aluminium oxide trihydrate (ATH) as filler the granite effect can be obtained by curable resin-containing structures such as slabs, bars etc., being fine ground to a granule and the granule being used in turn as a decorative filling material in the polymer phase. The granule is used according to the desired design in a particle size range selected for this. Fine granite designs can be obtained with particle sizes of 50-900 xcexcm, medium-coarse ones with 900-2500 xcexcm and coarse ones with  greater than 2500 xcexcm.
Since dark colour tones are frequently produced with granite designs, the reduction of the so-called water whitening is a central problem particularly with moulded parts. Due to the permanent influence of hot and cold water over long periods, a visibly unattractive brightening, the water whitening, comes about. It is already known that the water whitening can be reduced considerably by the use of fillers (for example ATH) which are coated with silanes. As a consequence, however, the mechanical and thermal properties of the moulded part material change, so that in the hot-cold cycle (cycle: 3 min hot water with 87xc2x0 C.; 3 min cold water with 21xc2x0 C.) the moulded parts already acquire small hair-line cracks after a few hundred cycles or even rupture completely, so that they have to be replaced.
A typical casting compound formulation consists in its main components of 40 to 55 wt % (silanized) ATH, 5 to 20 wt % granule and 30 to 40 wt % casting resin. If on the other hand unsilanized ATH is used, the crack behaviour in the hot-air cycle can certainly be rated as more positive, but on the other hand a serious brightening (water whitening) is observed on the areas of the moulded part washed by water.
The object of the invention is to propose a plastics moulding which in addition to the good resistance to water whitening possesses a high resistance to crack formation in hot-cold cycles.
This object is achieved according to the invention in the case of a plastics moulding of the kind described in the preamble by the fact that the polymer phase comprises up to 5 wt %, referred to the weight of the plastics moulding, of inorganic additive particles
that the particulate filling material is swellable in the curable reaction mass,
that the filled resin material comprises a content of inorganic filler in the range from 50 to 80 wt % with an average particle size of approx. 5 to approx. 100 xcexcm, and
that the filling material has a particle size in the range from 60 to 8000 xcexcm and is contained in the plastics moulding in a proportion of approx. 30 to approx. 75 wt %.
Due to the limitation of the inorganic additives in the polymer phase to not more than 5 wt % of the total weight of the moulding, the polymer phase remains substantially filler-free. The selection of the filling material according to its swellability in the reaction mass requires the abandonment of the manufacturing methods conventional to date in which the reaction mass has been pumped/poured into moulds together with the added filling material. The viscosity of the mixture of reaction mass and swollen filling material and optionally the inorganic additives attains a level such that the mixture is no longer pumpable or pourable.
Surprisingly it has been found, however, that precisely because of the swelling of the filling materials in the reaction mass a mixtures is obtainable [with which, when] it is charged into a mould half, on the closure of the mould and acting upon with a predetermined pressure, a void-free filling of the mould cavity is achievable without a destruction of the particulate structure of the filling material taking place, and the latter can exert unimpeded its decorative and/or contrasting effect on the exposed side, if this is desirable as with a granite design. A further aspect of the invention is to be seen in a plastics moulding of the kind described in the preamble in which a particulate filling material is used which is swellable in the reaction mass in such a way that a test formulation of 60 wt % of curable reaction mass and 40 wt % of the particulate filler exhibits a swelling factor xe2x89xa72.
There also forms part of the invention a process with which the plastics mouldings described above can be produced.
Such a process comprises the steps:
Production of a mixture of a curable reaction mass for the formation of a polymer phase with a particulate filling material and transfer of the mixture for curing into a mould, wherein
the curable reaction mass is used substantially free from inorganic fillers,
wherein there is used as filling material a filling material swellable in the reaction mass and containing a resin material filled with an inorganic filler,
wherein the content of the inorganic fillers in the filling material comes to 50 to 80 wt % and wherein the particles of the inorganic filler exhibit an average particle size of approx. 5 to 100 xcexcm and the filling material; exhibits a particle size of approx. 60 to approx. 8000 xcexcm and is so metered that it is contained in the plastics moulding in a content of approx. 30 to approx. 75 wt %,
wherein the filling material is allowed to swell in the curable reaction mass until such time as the mixture is no longer pourable,
wherein the mixture is placed for curing in one of the mould halves disposed in a press, the mould is closed and a predetermined compressive force is applied to the mould and the mixture is cured thermally in the closed mould.
The method of procedure according to the invention differs from the prior art both in the formulation and the technology.
In comparison with the conventional method of procedure the use of loose ATH in the polymer phase is abandoned substantially completely and instead of this a filling material is used which contains a resin material in the form of a polymer granule with a specified swelling behaviour and an adjusted content of an inorganic filler. There are considered as resin component all radically polymerisable casting resins, preferably however acrylic resins and polyester resins. There are suitable as fillers all mineral granular and fibrous materials, but also all synthetic granules, preferably however ATH. The filler content of the finished plastics moulding is markedly lower compared with the conventional technology at preferably approx. 40 to 50 wt %.
Due to the swelling of the filling material/polymer granules in the reaction mass for forming the polymer phase, mixtures which are no longer pumpable and which can be processed only with a special technology are obtained. The mixture consisting of reaction mass and filling material and optionally further additives is charged into the one half of the open mould and is by closing of the mould brought into the desired three-dimensional form in the mould press under corresponding pressure according to the displacement method.
Critical for the operation of the whole process according to the invention is a quite particular swelling behaviour of the filling material/polymer granules together with a particular final viscosity of the mixture.
The swelling behaviour of the polymer granules is dependent on the nature of the filler used, the filler content, the nature of the resin material used and the degree of crosslinking in the resin. There can be taken as a guide value in selecting a suitable filling material/polymer granule a swell factor with a value xe2x89xa72, which can be determined for a test formulation of 60 wt % reaction mass and 40 wt % filling material by the test method described in greater detail below.
If the swell factor is not maintained for the filling material used, a lack of flowability of the mixture frequently ensues and hence an unsatisfactory result in the moulding step.
Due to the high viscosity of the mixture of reaction mass and swollen filling material the problem of sedimentation, in particular of coarse granule particles, is completely eliminated. This means that an undesirable unequal distribution of the various components of the mixture in the finished plastics moulding is prevented. Accordingly there are no warpage problems in the case of slabs and there are smaller internal tensions in the case of moulded parts.
In the case of moulded parts the abandonment of loose ATH in the polymer phase and instead of this the substantially exclusive use of polymer granule as filling material improves the performance of the moulded parts considerably:
Whereas according to the prior art it was possible to obtain a satisfactory resistance to water whitening only at the expense of a much reduced hot-cold resistance, a good performance can now be obtained for both properties.
The reduced filler content of the plastics moulding results additionally in an improved abrasion, scratch and stain resistance. Due to the high viscosity in the mixture, there are no sedimentation problems of any kind. Even granite structures with particle sizes of 5 mm and more are realizable without further measures and without detracting from product quality.
Dispensing with loose ATH in the polymer phase leads in addition to a clear, attractive design image.
With slabs the dispensing with loose ATH in the polymer phase produces in addition to an improvement in the stain resistance improved crack strength under thermal loading. Because of the high viscosity there are no sedimentation problems involving coarse granule particles, and hence no warpage problems of any kind.
The dispensing with loose ATH leads to designs with a clear, attractive image.
The additives already discussed above which are possible in the polymer phase in small proportions (e.g. 0.1 to 5 wt %, preferably approx. 2 wt %, referred to the weight of the moulding in each case) can be for example a finely-divided, highly silanized, granular or fibrous filler, in particular e.g. extremely fine cristobalite meal with average particle sizes in the range from approx. 0.5 to 10 xcexcm, preferably approx. 3 xcexcm. The degree of silanization comes to preferably 0.1 to 2, preferably approx. 0.9 wt % (referred to the filler weight).
The use of such additives enables the depth effect of the water whitening effect to be further suppressed compared with formulations which dispense completely with additives in the polymer phase.
The curable reaction mass is frequently formed by a first monomer/pre-polymer syrup, which preferably has a pre-polymer content of approx. 5 to approx. 30 wt %. Such syrups are known per se from casting technology.
The viscosity of the first syrup and hence also of the reaction mass is preferably chosen in the range from approx. 20 to approx. 300 mPas, since a simple distribution of the reaction mass amongst the filling material and hence a uniform swelling of the filling material can be achieved in this way.
Experience shows that pre-polymers in the first syrup with an average molecular weight in the range from approx. 20 000 to 300 000 Da are suitable.
The first syrup contains preferably a radically polymerisable monomer or various monomers of this type and a pre-polymer as homo- or copolymer of the monomer or monomers. Particularly preferred here are monomers of the, acrylate type.
Alternatively the first syrup can also be based on a polyester resin.
The first syrup will preferably contain a cross-linking agent in an amount of approx. 0.5 to approx. 5 wt % (referred to the moulding weight), wherein the cross-linking agent is perferably chosen from the series of the bi- and polyfunctinal acrylate crosslinking agents. Examples of these are the cross-linking agents TRIM (trimethylolpropanetrimethacrylate) and EDMA (ethyleneglycol dimethacrylate).
Further a single-stage or multi-stage peroxide initiator system is preferably added to the syrup.
In order to facilitate the withdrawal of the fully cured plastics moulding from the mould, a mould release agent is preferably added to the first syrup in an amount of approx. 0.03 to approx. 0.3 wt % (referred to the moulding weight). Stearic acid in particular is suitable as mould release agent.
The resin material of the filling material is preferably produced with the use of a second monomer/pre-polymer syrup, wherein the pre-polymer content preferably comes to approx. 5 to approx. 30 wt % of the syrup. The statements on the suitable monomers and pre-polymers in conjunction with the description of the first syrup apply here analogously.
A value for the viscosity of the second syrup in the range from approx. 20 to approx. 300 mPas has proved to be particularly suitable, not only because the same material can then optionally be used for the first and the second syrup, but also because a mixing of the syrup with the filler and a complete wetting of the filler particles can be achieved particularly easily in this viscosity range.
The inorganic filler is preferably used in granular or fibrous form. In addition to a uniform filler a mixture of different fillers can also be used.
Examples of the filler include silicate fillers such as cristobalite, quartz or wollastonite, and also aluminium oxide and ATH.
Particularly preferred is ATH, which is silanized with a silane in an amount of approx. 0.04 to approx. 1 wt % (referred to the weight of the ATH).
With the second syrup, as with the first syrup, there is recommended the use of crosslinking agents, wherein the same recommendations apply as with the first syrup. The swelling behaviour of the filling material can be varied and brought into the desired range through the choice and the amount of the crosslinking agent added.
As regards the curing of the second syrup for forming the filled resin material, use is preferably made of an initiator system which has already been discussed in connection with the first syrup.
Not only is a single filled resin material suitable as particulate filling material, but the filling material can also be formed by a mixture of different resin materials. Thus filled and unfilled resin materials, for example, can be used in a mixture. The various resin materials can also contain different polymers and/or different fillers, wherein silanized and unsilanized fillers can also be included in the latter. In addition, the various resin materials can also contain various pigments. The various resin materials can further exhibit different particle sizes. The swellability of the various resin materials can naturally also vary in extent. Critical, however, is that the filling material as a whole shows the required swelling behaviour.
These and further advantages of the invention are explained in greater detail below by means of the examples.